Method for producing a hollow fiber membrane module or a capillary membrane module

ABSTRACT

The invention relates to a method for producing a hollow fiber membrane module or a capillary membrane module as well as a device having such a module. In the method, hollow fibers or capillaries are placed, in an unsintered state, in a mold structured for receiving the hollow fibers or capillaries and are sintered once inside the mold. Subsequently or simultaneously, the fibers are potted in the mold. The method permits producing hollow fiber membrane modules or capillary membrane modules in a simple manner with little risk of breakage.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a method for producing a hollow fibermembrane module or a capillary membrane module in which the hollowfibers or capillaries made of a ceramic or a material with a ceramiccontent are introduced into a mold structured for receiving the hollowfibers or capillaries and are potted in the mold with a pottingcompound. In addition, the present invention relates to a device havinga hollow fiber membrane module or a capillary membrane module producedaccording to the present method.

In the present invention, hollow fibers refers to pipe-shaped bodieswith an external diameter in the range from approximately >10 μm to 0.5mm, capillaries refers to bodies with an external diameter between 0.5and 3 mm.

The present method and device are primarily employed in filtration andseparation technology. In these technologies, among other things,inorganic membranes in the form of modules are utilized as separationtools for the filtration of liquids as well as the separation of gases.The hollow fiber membrane module or capillary membrane module producedwith the present method can be utilized for separation of, respectivelypurification of, gases and vapors, in particular in high-temperatureapplications, as well as for the filtration of liquids inmicro-filtration, ultra-filtration and nano-filtration and as a membranereactor.

2. Prior Art

EP 0 941 759 A1 discloses a method for producing a hollow fiber membranemodule in which the sintered hollow fibers are introduced into a moldand are potted in this mold with a potting compound. Aceramic-containing compound is used as the potting compound, which isthen hardened respectively solidified in a suited thermal step. The moldfor receiving the hollow fibers is designed as a perforated plate, whichthen is fitted with the fibers potted inside it into a housing.

Production of a hollow fiber membrane module according to the method ofthis printed publication is, however, difficult as the sintered hollowfibers have a great propensity to break as is characteristic ofceramics. It is not easy to handle such type sintered hollow fibers sothat insertion into the openings of the mold designed as a perforatedplate is difficult and can lead to the hollow fibers breaking.

EP 0 938 921 A1 discloses another method for producing a hollow fibermembrane module. In this method, a bundle of sintered hollow fibers isplaced in a cylindrical mold and is potted in this mold while thepotting compound is impinged with ultrasound.

However, in this method too the hollow fibers can break very easily.

The object of the present invention is to provide a method for producinga hollow fiber membrane module or a capillary membrane module as well asa device having such a module, which is easy to produce and with lessrisk of the hollow fibers or capillaries breaking.

SUMMARY OF THE INVENTION

The object is solved with the method and the device according to theclaims. Advantageous embodiments of the method and the device are thesubject of the subclaims.

In the present method for producing a hollow fiber membrane module orcapillary membrane module, hollow fibers or capillaries made of ceramicor a material with a ceramic content in an unsintered state, i.e. asgreen fibers, are placed in a mold structured for the reception ofhollow fibers or capillaries. The hollow fibers or capillaries are notsintered until once inside this mold in a thermal process step. Thehollow fibers or capillaries are potted, either before or aftersintering, with a potting compound, which connects them with the mold.This casting process is familiar to someone skilled in the art under theterm potting, the casting compound is called the potting compound. Thepotting compound is then hardened respectively solidified in such amanner that a hollow fiber membrane module respectively a capillarymembrane module is created which can be placed in a housing and utilizedin technical systems.

Solidification of the potting compound may occur, for example by meansof a thermal process step. If a ceramic material is employed as thepotting compound sintering of the hollow fibers respectively thecapillaries and hardening can, therefore, occur in the same thermalstep. This technique is referred to as cofiring.

In the present method, the hollow fibers respectively the capillariesare not inserted into respectively not placed in a structured mold in asintered state but in a green state. This mold is then part of thehollow fiber membrane module respectively of the capillary membranemodule and is shaped in such a manner that the hollow fibersrespectively capillaries can be received in it. The mold can, forexample, be made of a porous ceramic or other inorganic materials, suchas for example metal or glass. Exemplary versions of preferredembodiments are grooved or corrugated, plate-like bodies or star-shapedbodies which, due to their geometry, have recesses to receive the fibersor capillaries. Placing the fibers in the mold can occur manually ormechanically. After being placed in the mold, the green fibers aresintered and potted.

There are various possible available methods for potting, such as forexample spinning, casting or tampon pressure. After the pottingcompound, preferably a ceramic or a polymer, has hardened, the hollowfiber ends respectively the capillary ends are cut off in such a mannerthat the lumina of the fibers are open. Cutting off the fiber ends canoccur by means of a suited severing method, for example using a diamondwire saw, by means of water jet blasting technology or by means of lasercutting technology.

With the proposed method, production of hollow fiber membrane modules orcapillary membrane modules can be significantly simplified. Handling ofthe green fibers and placing them in a mold are rendered significantlyeasier and lead to far less breakage than when placing sintered fibersin a mold designed, for example, as a perforated plate respectivelyinserting a bundle of sintered fibers into a cylindrical body. Itsurprisingly turned out that the ceramic hollow fibers respectivelycapillaries do not connect during sintering but rather remain separate.It is this surprising teaching of the inventors that makes producing ahollow membrane module or a capillary membrane module with the presentmethod possible.

With this method fiber breakage and other defects can be preventedduring production of the module. The fibers can, for example, be placedin bundles into the structured mold. During the subsequent drying andsintering process, distortion respectively fraying of the fibers isprevented due to the external molding by the mold.

In addition to this, using a ceramic potting compound obviates utilizingtwo separate thermal treatment steps. But rather sintering the hollowfibers respectively the capillaries and solidifying respectivelysintering the ceramic potting compound can occur in the same thermaltreatment step. In this case, the thermal expansion coefficients of thematerials for the hollow fibers respectively for the capillaries and thepotting compound must be matched in order to prevent the development ofexcessive mechanical tension.

The hollow fibers or capillaries can be provided in a prior art manneras green fibers respectively in a green state. They can be obtained byspinning respectively by extrusion of inorganic or metal organiccompounds, such as preliminary polymer stages or inorganicbinder-containing suspensions of aqueous solutions of salts or of powderfilled sol/gels. Hollow fibers or capillaries produced in this mannerare flexible and easy to handle in a green state. In a sinteredrespectively pyrolyzed state, the hollow fibers or capillaries, such asemployed in the present method can be composed of oxidic substances suchas ZrO₂, TiO₂, α-Al₂O₃, γ-Al₂O₃, 3Al₂O₃*2SiO₂ (mullite), MgAl₂O₄(spinel), SiO₂, of perowskites, hydroxylapatite, zeolites, nonoxidicsubstances such as SiBNC, Sic, BN, Si₃N₄, C, as well as of metal such ascopper, titanium, iron, special steels or transition metal alloys. Thislist is, of course, incomplete. Someone skilled in the art is familiarwith suited materials for producing ceramic hollow fibers orcapillaries.

The material of the potting compound can be composed of the samematerial as the hollow fibers respectively the capillaries or of anothersuited inorganic or organic material. The expansion coefficients ofhollow fibers or capillaries and the potting compound should match.Preferably, the difference in expansion coefficient should be notgreater than 5×10⁻⁶K⁻¹. A higher thermal expansion coefficient of thefiber material can be tolerated more readily than the reverse.

The expansion coefficient of prior art materials for the pottingcompound respectively for the material for the hollow fibersrespectively for the capillaries are 8×10⁻⁶K⁻¹ for Al₂O₃, 10×10⁻⁶K⁻¹ forZrO₂(Y₂O₃ stabilized), 0.5×10⁻⁶K⁻¹ for SiO₂, 8−10×10⁻⁶K⁻¹ for TiO₂,4.5×10⁻⁶K⁻¹ for SiC. From these examples it is evident that there arenumerous materials that meet the above condition of a minor differencein thermal expansion coefficients.

In addition to a thermal process step, for example drying,solidification of the potting compound to the corresponding green statecan occur by changing the surface charge of the ceramic powder particlesof this potting compound. Such a change in the surface charge can occur,for example, by means of enzymatic release of protons or hydroxyl ions.

For the intended use of the module produced according to the inventedmethod as a technical process device in a system for liquid filtrationor gas separation, the mold with the hollow fibers respectively thecapillaries potted in it are placed in a suited housing. The geometry ofthe mold containing the hollow fibers respectively the capillaries andthe housing are matched in such a manner that a so-called feed space forfeeding the to-be-filtered medium and a so-called permeate space for thefiltrate are formed, which are separated from each other in a gas tightmanner by seals disposed in the housing outside the membrane arearespectively filter area, which is formed in a prior art manner by theside walls of the hollow fibers respectively of the capillaries. Thehousing may be executed as a tight ceramic or as a metal cartridgesystem.

In an alternative preferred embodiment, the green fibers, mold and aceramic housing can be potted in a single step with a suited pottingcompound of ceramic and sintered together (cofiring). In this method ofproduction, the relationship between size of the mold and the length ofthe green fibers must be matched in such a manner that shrinking of thefibers during subsequent sintering is taken into account.

In addition to this, a module element produced in this manner can beprovided with further ceramic coatings, such as for example of α-Al₂O₃,γ-Al₂O₃, MgAl₂O₄, TiO₂, ZrO₂, etc. with metal or metal alloy coatings,such as for example of transition metals from the groups 4-6, 10, 11, inparticular of alloys containing these transition metals, lave phases,metallic glasses or polymers such as for example polyimide. Thisadditional coating permits obtaining gas separation membranes and/orincreasing potting density.

A hollow fiber membrane module or capillary membrane module producedwith the present method respectively with the proposed device with sucha type module can be used in many technical fields of application.Examples are drying or moisturizing air (air conditioning), catalysis,cleaning hot gases, separation of gases, pervaporation, vaporpermeation, heterogeneous catalysis, use in membrane reactors, in heatexchangers, in contactors, in fuel cells, as prefilters forpurification, filtration of aggressive media such as hot acidic mediaand lyes or solvents, filtration of abrasive, toxic, microbiologicallyor otherwise contaminated liquids and the reprocessing of emulsions.

In the preceding description it was already made apparent that variousfiber/potting compound material combinations can be employed. Therefore,both oxidic and nonoxidic respectively metallic materials can be used asthe hollow fiber material respectively as the capillary material and asthe potting material. Hollow fibers respectively capillaries(hereinafter for the sake of simplicity only referred to as fibers) andthe potting may be composed of the same materials or of differentmaterials. The porosity of the potting material must be less than theporosity of the fibers. In the case of a combination of a ceramicmaterial for the fibers and a polymer for the potting, oxidic andnonoxidic respectively metallic materials can be used as the fibermaterial. In this case, polymers or organic materials, such as forexample epoxy resins (filled and not filled systems) or silicones can beemployed as potting materials. Here too, the porosity of the pottingmaterials must be less than the porosity of the fibers.

Preferably, the receiving mold also has the same or a similar thermalexpansion coefficient as the fibers and the potting material. In thismanner, tensions are prevented during production of the module and inits subsequent use at high temperatures.

The proposed device comprises a module produced according to the presentmethod in a housing. The mold with the fibers and the housing areadapted geometrically in such a manner that a feed space and a permeatespace are formed which are separated gas tight from each other by sealsor a sealing material between the mold and the housing but not withinthe membrane area. The housing has openings for the lumen inlet andlumen outlet of the fibers and for the exterior inlet and exterioroutlet to respectively from the interior of the mold, which arepreferably designed as connections. In the same manner the mold formsopenings which permit feeding respectively removing gas or liquid viathe corresponding exterior openings of the housing to the exterior wallsof the fibers.

The housing with the fitted mold is preferably designed as a cartridgesystem, with the mold and the cartridge being sealed in a stuffingbox-like manner between the lumen side and the exterior side of thefibers. The cartridge serves to adapt the device to the overall system.The interior of the receiving mold is accessible via the describedopenings in the periphery of the mold. The housing is preferably made ofa metallic material.

BRIEF DESCRIPTION OF THE DRAWINGS

The method and the device are briefly described in the following usingpreferred embodiments with reference to the accompanying drawingswithout the intentions of limiting the scope or spirit of the inventiveidea.

FIG. 1 shows a first example of a mold with hollow fibers placed in it;

FIG. 2 shows a second example of a mold with potted hollow fibers;

FIG. 3 shows a third example of a mold with potted hollow fibers;

FIG. 4 shows an example of a preferred embodiment of the present deviceas a cartridge system.

WAYS TO CARRY OUT THE INVENTION

FIGS. 1-3 show different examples of preferred embodiments of a mold 1,as is utilized in the present method. In the example of FIG. 1, mold 1is executed as a grooved plate made of ceramic or metal. Hollow fibers 3in a green state are placed, preferably in bundles, into the grooves 2of the mold, as the figure shows. Following or simultaneously with thesubsequent potting step, these fibers 3 are connected with mold 1.

The mold (1) of FIG. 2 is designed star-shaped and preferably made of aporous ceramic or porous metal. The hollow fibers 3 in a green state areplaced in the recesses 4 formed by the star shape, sintered andsimultaneously or subsequently potted.

FIG. 3 shows a third example, in which mold 1 is formed by two platesprovided with hole openings 5. The green fibers are placed in theseopenings 5 and then sintered and potted. In this manner a module in theform of a multi-channel is formed, as FIG. 3 shows.

In all three examples, after the potting step fibers 3 are cut off wheretheir ends protrude from mold 1 in order to expose the fiber lumen.

Finally FIG. 4 shows an example of a preferred embodiment of the presentdevice in the form of a cartridge system for insertion in a system forfiltering liquids or for gas separation. The capillary membrane moduleor hollow fiber membrane module produced with the present methodcomprises in this example a cylindrical mold body 1 made of glass orceramic. In this mold 1, the ceramic hollow fibers or capillaries 3 arepotted at their ends. The potting compound bears the number 6. Thematerial of mold 1 ideally has the same or a similar expansioncoefficient as the ceramic hollow fibers respectively capillaries 3 sothat no thermal tension develops.

Mold 1 bears in its periphery in the region of its front end and rearend in at least one access opening 7 respectively via which the liquidsreach the interior of the mold and in this manner the outer sides of theceramic hollow fibers or capillaries 3 respectively can be removed fromthe interior of mold 1. A cartridge 10 which is preferably made of metalor a high-temperature stable plastic bearing all the connections for thelumen inlet and lumen outlet as well as the exterior inlet and exterioroutlet is pushed over this mold 1. The connection openings for theexterior inlet and exterior outlet 8 are located exactly over the accessopenings 7 of the mold.

The connections may be executed as threads or as welded hexagon nipples.The lumen connections 9 are attached to the housing 10 at the front andprovide access to the lumen of the hollow fibers respectivelycapillaries 3. These connections are also preferably designed as threadsor welded hexagon nipples. The diameter of mold 1 is adapted to theinner diameter of cartridge 10 in such a manner that there is onlyminimal play between the two. Preferably the two are connected in a formfitting manner. Seals 11, preferably square washers or O-ring seals, areprovided between mold 1 and the inner walls of cartridge 10, on the onehand, in order to seal between the lumen inlet and lumen outlet as wellas the exterior inlet and exterior outlet. The washers permit, on theother hand ensuring sealing between the exterior inlet and exterioroutlet. Washers 11 may, for example, be made ofhigh-temperature-tolerant polymers which are tolerant of hightemperatures and preferably also tolerant of chemicals, such as forexample polyimides, PTFE, Viton®, Kalrez®, silicon or of graphite ormetals.

In addition to washers, the entire intermediate space between the moldbody 1 and the cartridge 10 may also be filled with sealingmaterial—with the exception of the inlet and outlet openings 7, 8 forthe interior of the mold body 1—in such a manner that there is onlyminimum dead space.

Generating pressure force on the washers 11 can, for example, occur viaswivel nuts, tie rods, or screws, with the thermal motion being offsetvia spring systems. The figure shows the use of a press plate 12 bymeans of which a pluglike designed body 13 is pressed on the outerwashers 11 between mold body 1 and cartridge 10. The two press plates 12are held together by threaded rods 14 with nuts.

A cartridge system as shown in FIG. 4 has the advantage that no thermaltension occurs in the capillaries or hollow fibers 3 from the receivingmold 1, and it simultaneously permits simple adaptation of the module toan entire system, for example a production system, via pipe connections.

Three examples of variants of preferred embodiments of the method forproducing a hollow fiber membrane module are given in the following.

In the first example, α-Al₂O₃ green fibers are utilized, which wereproduced according to the lycocell method of DE 44 26 966 A1. The fibersare placed in a green state in bundles on a corrugated mold made ofporous Al₂O₃ ceramic, as for example shown in FIG. 1. The green fibersare sintered in this mold at 1450° C. After the sintering step, the endsof the fibers are potted (static potting) with a two-componentepoxy-based adhesive compound (Biresin, hardener HM, SIKA Chemie) andthe potting compound is hardened in air for 24 hours. Following this,the hollow fibers are cut together with the mold in such a manner thatthe lumina of the fibers are open. A plurality of such molds with hollowfibers are transferred into a housing, with plastic seals separatingfeed and permeate spaces gas-tight.

In a second method, ZrO₂ green fibers that were also produced accordingto the lycocell method are utilized. The fibers are placed in a greenstate in bundles in a star-shaped mold made of Al₂O₃ceramic in such amanner that the green fibers protrude beyond the boundary of the mold.The degree of protrusion of the green fibers is selected to compensatefor the degree of shrinking during sintering (cf. FIG. 2). The insertedgreen fibers are sintered in the mold at 1200° C. Following thesintering step, the ends of the fibers are potted (static potting) witha silicon adhesive compound (Silicone AP, Dow Corning) and the pottingcompound is hardened in air for 24 hours. Then the hollow fibers are cuttogether with the mold in such a manner that the lumina of the fibersare open. The mold with the hollow fibers is then conveyed into ahousing, with plastic seals separating the feed space from the permeatespace.

In the last example of a preferred embodiment, α-Al₂O₃ green fibers areproduced according to the Monsanto method (DE 2919560 A1). The fibersare placed in a green state in bundles on a corrugated Al₂O₃ ceramic, asshown in FIG. 1. The green fibers are sintered in the mold at 1450° C.After sintering, the ends of the fibers are potted with a ceramicpotting compound. The potting compound has the following composition:

-   -   1180 g Al₂O₃ (CL 370 C Alcoa)    -   1.76 g 4,5-dihydroxy-1,3 benzol disulfonic acid    -   3.54 g uric acid    -   109 g bidistilled water.

Just before casting, 2000 units of urease (EC 3.5.1.5) are added, whichleads to rapid hardening of the potting compound.

After potting, the system is calcinated at 1450° C. After this, thehollow fibers are cut together with the mold in such a manner that thelumina of the fibers are open. A plurality of such molds containinghollow fibers is transferred into a housing with temperature stableseals, for example made of graphite, metals or HT polymers separatingthe feed space and the permeate space gas-tight. Yielded is a hollowfiber module which is suited for use at high temperatures.

If the ceramic potting compound is composed in such a manner that theshrinking rates of the fibers and the potting compound are approximatelythe same, after placing the green fibers in the mold the fibers can bepotted immediately in a green state. Fibers and potting compound arethen sintered in the mold in a single thermal treatment step (cofiringprocess).

LIST OF REFERENCE NUMBERS

1 mold respectively mold body

2 grooves

3 hollow fibers respectively capillaries

4 recesses

5 hole openings

6 potting compound

7 access openings of the mold

8 connections for exterior inlet and outlet

9 lumen connections

10 housing respectively cartridge

11 seals

12 press plates

13 plugs

14 threaded rods with nuts

1. A method for producing a hollow fibers membrane module or capillariesmembrane module, in which the hollow fibers or the capillaries arecomposed of a ceramic material or a material including a ceramiccontent, comprising placing said hollow fibers or said capillaries in anunsintered state in a mold structured for receiving said hollow fibersor said capillaries, connecting said hollow fibers or said capillarieswith said mold by potting with a potting compound, and sintering saidhollow fibers or said capillaries in said mold.
 2. A method according toclaim 1, wherein the potting compound comprises a material including aceramic content.
 3. A method according to claim 2, wherein saidsintering of said hollow fibers or said capillaries and hardening ofsaid potting compound occur in a common thermal processing step.
 4. Amethod according to claim 3, wherein material for said potting compoundand the material for said hollow fibers or said capillaries are selectedto have a thermal expansion coefficient which differs by less than5*10⁻⁶ K⁻¹.
 5. A method according to claim 2, wherein material for saidpotting compound and the material for said hollow fibers or saidcapillaries are selected to have a thermal expansion coefficient whichdiffers by less than 5*10⁻⁶ K⁻¹.
 6. A method according to claim 5,wherein the thermal expansion efficient of said material for said hollowfibers or said capillaries is equal to or greater than the thermalexpansion coefficient of said material for said potting compound.
 7. Amethod according to claim 2, wherein said ceramic content of saidpotting compound includes powder particles, and said method furthercomprises solidifying said potting compound by altering a surface chargeof the powder particles.
 8. A method according to claim 1, wherein thepotting compound comprises a polymer or organic material.
 9. A methodaccording to claim 1, wherein said hollow fibers or said capillaries areplaced in bundles in said mold.
 10. A method according to claim 1,wherein said mold comprises porous ceramic or inorganic material.
 11. Amethod according to claim 1, wherein said mold includes elongatedrecesses for reception of said hollow fibers or said capillaries.
 12. Amethod according to claim 1, further comprising attaching said moldcontaining said hollow fibers or said capillaries to a housing by meansof at least one seal in order to form a feed space and a permeate spacewhich are separated from one another in a gas-tight manner by said atleast one seal.
 13. A method according to claim 12, wherein said pottingof said hollow fibers or said capillaries in said mold occurs in saidhousing with said potting compound, and said sintering of said hollowfibers or said capillaries in said mold occurs following said attachingof said mold to said housing and as a thermal processing step to sintersaid hollow fibers or said capillaries and to harden said pottingcompound.
 14. A method according to claim 1, wherein said mold with saidhollow fibers or said capillaries and said potting compound are providedwith at least one coating.
 15. A device having a hollow fibers membranemodule or a capillaries membrane module provided according to one ofclaims 1 to 11 or 30 comprising, a housing in which said mold of saidhollow fibers membrane module or said capillaries membrane module isdisposed in said housing in such a manner that a feed space and apermeate space are formed, wherein said housing includes a lumen inlet,a lumen outlet, an exterior inlet and an exterior outlet, and a sealbetween each of said feed space and said permeate space, said lumeninlet and said lumen outlet, and said exterior inlet and said exterioroutlet, each said seal being present between said mold and said housing.16. A device according to claim 15, wherein said housing is designed asa metallic cartridge.
 17. A device according to claim 15, wherein saidmold is cylindrical in shape, is held in said housing by at least oneplug inserted at a front end of said housing, and is pressed to a tightfit by each said seal disposed at the front end between said mold andsaid housing.
 18. A device having a hollow fibers membrane module or acapillaries membrane module provided according to claims 12 or 13,wherein said housing includes a lumen inlet, a lumen outlet, an exteriorinlet, and an exterior outlet, and a seal between each of said feedspace and said permeate space, said lumen inlet and said lumen outlet,and said exterior inlet and said exterior outlet, each said seal beingpresent between said mold and said housing.
 19. A device according toclaim 18, wherein said housing is designed as a metallic cartridge. 20.A device according to claim 18, wherein said mold is cylindrical inshape, is held in said housing by at least one plug inserted at a frontend of said housing, and is pressed to a tight fit by each said sealdisposed at the front end between said mold and said housing.